The present invention relates to a predistortion-type distortion compensation amplifying apparatus suitable for use in a high-frequency circuit for mobile communication.
In mobile communication, a digital modulation system is used to increase the efficiency of frequency utilization, etc. In such case, distortion caused by non-linearity of characteristics of a power amplifier disturbs the adjacent channel, which is a problem. In order to prevent disturbance to the adjacent channel, a power amplifier having a low adjacent channel power (ACP: Adjacent Channel Power) is required. However, a use of the power amplifier within a linear operation region is not always advisable in view of the circuit scale and cost. Instead, a predistortion (hereinafter also referred as distortion compensation) is used in many cases.
Predistortion is a method of beforehand distorting an input signal to be amplified using a function representing a reverse characteristic of an input-output characteristic of the amplifier (power amplifier: hereinafter, the power amplifier being simply referred as an amplifier occasionally) when the signal is inputted to the amplifier. Namely, predistortion is a technique of beforehand distorting an input signal to be amplified and amplifying it, whereby a signal which has been linearized appears at the output of the amplifier.
FIG. 18 is a diagram showing an example of the radio transceiver using predistortion. In a radio transceiver 50 shown in FIG. 18, a baseband signal to be transmitted is modulated and predistorted in a digital signal processor (DSP: Digital Signal Processor) 50a, where a distortion compensation coefficient operating process is performed to estimate non-linear distortion of a power amplifier 50c. In a quadrature modulating-demodulating unit 50b, the processed baseband signal is up-converted into the RF (Radio Frequency) band. In the power amplifier 50c, a predetermined power is applied to the signal, and the signal is fed to an antenna 50e via a synthesizer 50d and transmitted.
On the other hand, the modulated signal fed back from a part of the signal outputted from the power amplifier 50c is down-converted into a baseband signal having a distorted component in the quadrature modulating-demodulating unit 50b. The converted signal is inputted to the digital signal processor 50a, where a distortion compensation coefficient operating process is performed. Accordingly, an RF signal free of distortion is outputted from the antenna 50e by this loop process. In the structure shown in FIG. 18, predistortion is performed in the baseband. Signal processing in the baseband will be now described with reference to FIG. 19, corresponding to equations.
FIG. 19 is a diagram showing an example of a known predistortion circuit (also referred as a predistorter). A quadrature modulator 60 shown in FIG. 19 performs the distortion compensation coefficient operating process, which comprises memories 61 and 62, multipliers 63a, 63b, 63c and 64d, and adders 64a and 64b. Baseband signals I and Q are undergone the distortion compensation operating process by referring to the respective memories 61 and 62 (memory regions denoted by reference characters 61a, 61b, 61c and 61d, and memory regions denoted by reference characters 62a, 62b, 62c and 62d) in a software process or the like. The signals processed in these memories 61 and 62 are multiplied in the multipliers 63a-63d, added in the adders 64a and 64b, and Ipd and Qpd are outputted.
When an output of an amplifier is Po(t), the output is expressed by a product of a function f(t) of amplitude and a function g(t) of phase as shown in Equation (1):
Po(t)=f{Mi(t)}xc2x7exp[(xe2x88x92jxc2x7g{Mi(t)})xc2x7exp(xcfx89t)xe2x80x83xe2x80x83(1) 
where, Mi(t) is magnitude of amplitude of a modulated wave, xcfx89 is a center frequency, t is a time, and j is an imaginary unit which represents j2=xe2x88x921.
When input signals to the quadrature modulator 60 are I(t) and Q(t), magnitude x(t) of amplitude of a modulated wave inputted to the amplifier are expressed by Equation (2) (operation in the memory regions 61a and 62a shown in FIG. 19):
X(t)={square root over ((I(t)2+Q(t)2))}xe2x80x83xe2x80x83(2) 
Magnitude y(t) of amplitude of a modulated wave component outputted from the amplifier is expressed, with G being a gain, by Equation (3):
y(t)=Gxc2x7x(t)xe2x80x83xe2x80x83(3) 
FIG. 12 is a diagram showing an example of the input-output characteristic of the amplifier. A part denoted by B1 in FIG. 12 is a non-saturation region, whereas a part denoted by B2 is a saturation region. The input-output characteristic changes its characteristic at a point denoted by A, which is a function having an upper limit because of the saturation characteristic of the amplifier. In order to compensate the distortion of an output signal amplified as expressed by Equation (3), distortion compensation using an inverse function fxe2x88x921(t) of the function f(t) of amplitude within the quadrature modulator 60 is performed. An output Ppd(t) of the amplifier undergone the distortion compensation is expressed by Equation (4):
Ppd(t)=fxe2x88x921(y)xc2x7exp[jxc2x7g{fxe2x88x921(y)}]xc2x7exp(xcfx89t)xe2x80x83xe2x80x83(4) 
Namely, Ipd(t) and Qpd(t) which are deformed I(t) and Q(t) (hereinafter abbreviated as Ipd and Qpd, respectively) are expressed by Equations (5) and (6), respectively:
Ipd={fxe2x88x921(y)/x}xc2x7[I cos [g{fxe2x88x921(y)}]xe2x88x92Q sin [g{fxe2x88x921(y)}]]xe2x80x83xe2x80x83(5) 
Qpd={fxe2x88x921(y)/x}xc2x7[Q cos [g{fxe2x88x921(y)}]xe2x88x92I sin [g{fxe2x88x92(y)}]]xe2x80x83xe2x80x83(6) 
where x(t) and y(t) are abbreviated as x and y. Generally, input-output relationships of the Equations (5) and (6) are stored in the memories 61 and 62 as a reverse characteristic of the characteristic as shown in FIG. 12. Values of I(t) and Q(t) are, for example, very frequently referred at sampling time intervals for digital signals, and the outputs Ipd and Qpd are obtained.
Namely, y is determined from data in the memory region 61b shown in FIG. 19, then fxe2x88x921(y)/x in the Equations (5) and (6) is calculated from y and data in the memory region 61c. Further, {fxe2x88x921(y)/x}xe2x80xa2I in the Equation (5) and {fxe2x88x921(y)/x}xe2x80xa2Q in the Equation (6) are calculated from data in the memory region 62b, and outputted. Similarly, y is determined form data in the memory region 62b, then g{fxe2x88x921(y)} in the Equations (5) and (6) is calculated from y and data in the memory region 62c. Further, cos [g{fxe2x88x921(y)}] and sin [g{fxe2x88x921(y)}] in the Equations (5) and (6) are calculated from data in the memory region 62d, and outputted. These outputs are added in the multipliers 63a-63d, after which, added in the adders 64a and 64b, and deformed Ipd and Qpd are outputted. As this, distortion compensation is performed using a predistortion circuit as the quadrature modulator 60 to make the amplification characteristic of the amplifier linear, in general.
Incidentally, as a method for compensating a frequency characteristic in the vicinity of higher harmonic in bias circuits of the amplifier and the power circuit (not shown), and a matching circuit, there has been proposed a method of determining a coefficient from a differential value or an integral value of an amplitude quantity of an input signal, and multiplying the original signal by the coefficient to obtain a predistortion signal.
FIG. 20 is a diagram showing an example of a predistortion circuit using a differential value or an integral value of an input signal. In a predistortion circuit 70 shown in FIG. 20, magnitude of amplitude of a demodulated wave with respect to input signals I and Q is calculated in an amplitude calculating unit 70a, fxe2x88x921(y) is calculated in an inverse function calculating unit 70b, a differential value or an integral value of an amplitude quantity of the input signals is calculated in a differential/integral coefficient information adding unit 70c, an appropriate coefficient is obtained in a coefficient table 70d, the coefficient is multiplied in each of multipliers 70e and 70f, then Ipd and Qpd are outputted.
However, when linearlization by Equation (3) is performed in the circuits shown in FIGS. 19 and 20, distortion increases. FIG. 23 is a diagram showing an input-output characteristic of a linearized amplifier. Since the input-output characteristic becomes discontinuous at a point (refer to a point A) where the output changes to the saturation region as shown in FIG. 23, distortion increases when the input signal steps into this region, which is a problem. This distortion will be now described with reference to FIGS. 21(a), 21(b), 22(a) and 22(b).
FIG. 21(a) is a diagram showing an example of a signal wave in the case where two waves having an equal amplitude are inputted. FIG. 21(b) is a diagram showing a spectrum of the signal shown in FIG. 21(a). As shown in FIG. 21(b), this signal has components of only frequencies f1 and f2.
FIG. 22(a) is a diagram showing an example of a signal waveform in the case where the signal shown in FIG. 21(a) is amplified by an amplifier having the input-output characteristic shown in FIG. 23. FIG. 22(b) is a diagram showing a spectrum of the signal shown in FIG. 22(a). As shown in FIG. 22(b), there is a problem which a spurious signal generates over a wide range of the frequency.
Moreover, since the frequency of reference to the memory is equal to a sampling time for the signal, the memory reference cannot catch up with in the case of a signal changing at a high speed, which leads to a difficulty in applying it to a high-speed signal. There is another problem which a predistortion circuit as shown in FIG. 19 generates an error in a coefficient to be referred when the characteristic varies due to a change in temperature or a change with time of the amplifier.
In the light of the above problems, the present invention provides a predistortion-type distortion compensation amplifying apparatus not requiring an external RF circuit having a linear amplification characteristic such as a feed-forward-type amplifier, which can decrease a spurious signal by adding an odd-power component to an input signal to make the amplifier operate within a range not exceeding the saturation region of the amplifier, decrease the adjacent channel power by multiplying the odd-power component of the input signal by a coefficient (coefficient information) and adding the result to the input signal, and perform a distortion compensating operation even if the frequency of reference to the memory is decreased.
A predistortion-type distortion compensation amplifying apparatus according to this invention comprises an amplifier for amplifying a signal to be transmitted, and a signal processing unit provided on the front side of the amplifier to perform a coefficient changing process to change coefficient information of a power component of an amplitude quantity contained in the signal to be transmitted, and being operable to output a processed signal.
Accordingly, it is possible to decrease spurious signals generating in a wide range of a frequency, and decrease the adjacent channel power. Additionally, even if characteristics of the amplifier vary due to a change in temperature or a change with time, it is possible to cope with it by comparing a part of an output signal and a part of an input signal, calculating an error therebetween, and rewriting a coefficient table when the error is large.
The signal processing unit performs a process with a function representing a reverse characteristic of an input-output characteristic of the amplifier on the signal to be transmitted, performs a coefficient changing process, and makes an odd-order component appear in an output signal of the amplifier, and the amplifier operates in a non-saturation region.
Accordingly, it is possible to prevent spurious from spreading over a wideband, and perform distortion compensation without an external RF circuit having a linear amplification characteristic. It is also possible to contribute to a reduction of the power consumption and allow compactness of the circuit scale. Further, a frequency characteristic in the vicinity of higher harmonic in a bias circuit of the amplifier, a bias circuit of the power circuit or a matching circuit, which leads to improvement of the accuracy.
The signal processing can decrease contribution of a power component of odd order of an amplitude quantity contained in the signal to be transmitted when performing a coefficient changing process.
Accordingly, the adjacent channel power which is a problem in a spurious signal is decreased, an external RF circuit having a linear amplification characteristic becomes unnecessary, and the circuit scale can be reduced, which leads to contribution to a reduction of the power consumption and allows compactness of a portable telephone or the like. Frequency of reference to the coefficient table becomes equal to a time for switching an average output power, it is thus possible to form the circuit with a memory having a relatively slow access speed.